Cardiovascular diseases are the leading cause of death, with atherosclerosis being the leading cause of cardiovascular diseases. Atherosclerosis is a disease of the arteries and is responsible for coronary heart disease associated with many deaths in industrialized countries. Several risk factors for coronary heart disease have now been identified: dyslipidemia, hypertension, diabetes, smoking, poor diet, inactivity and stress. Dyslipidemia is elevation of plasma cholesterol (hypercholesterolemia) and/or triglycerides (TGs) or a low high-density lipoprotein (HDL) level that contributes to the development of atherosclerosis. It is a metabolic disorder that is proven to contribute to cardiovascular disease. In the blood, cholesterol is transported in lipoprotein particles, where the low-density lipoprotein (LDL) cholesterol (LDL-C) is considered “bad” cholesterol, while HDL-cholesterol (HDL-C) is known as “good” cholesterol. Lipid and lipoprotein abnormalities are extremely common in the general population and are regarded as a highly modifiable risk factor for cardiovascular disease, due to the influence of cholesterol on atherosclerosis. There is a long-felt significant unmet need with respect to CVD with 60-70% of cardiovascular events, heart attacks and strokes occurring despite the treatment with statins (the current standard of care in atherosclerosis). Moreover, new guidelines suggest that even lower LDL levels should be achieved in order to protect high risk patients from premature CVD (1).
The establishment of a link between PCSK9 and cholesterol metabolism was rapidly followed by the discovery that selected mutations in the PCSK9 gene caused autosomal dominant hypercholesterolemia (2), suggesting that the mutations confer a gain-of-function (3) by increasing the normal activity of PCSK9. This was supported by the experiment in which wild-type and mutant PCSK9 (S127R and F216L) were expressed at high levels in the livers of mice; hepatic LDLR protein levels fell dramatically in mice receiving either the wild-type or mutant PCSK9 (4, 5). No associated reductions in LDLR mRNA levels were observed, indicating that overexpression of PCSK9, whether mutant or wild-type, reduces LDLRs through a post-transcriptional mechanism.
Given that gain-of-function mutations in PCSK9 cause hypercholesterolemia, it was reasonable to ask if loss-of-function mutations would have the opposite effect and result in hypocholesterolemia. Three loss-of-function mutations in PCSK9 (Y142X, L253F, and C679X) were identified in African-Americans (6). These mutations reduce LDL-C levels by 28% and were shown to decrease the frequency of CHD (defined as myocardial infarction, coronary death or coronary revascularization) by 88%. Rashid et al. (7) studied the mechanism of loss-of-function mutations in mice where PCSK9 was inactivated. They reported that these knockout mice showed increased hepatic LDLR protein (but not mRNA), increased clearance of circulating lipoproteins and reduced plasma cholesterol levels. Structure-function relationship analysis of the naturally occurring mutations in PCSK9 has also provided insights into the mechanism of action of PCSK9. Interestingly, mutations in PCSK9 that were found to be associated with the greatest reductions in LDL-C plasma levels are those that prevent the secretion of mature PCSK9 by disrupting its synthesis (Y142X), autocatalytic processing (L253F), or folding (C679X) (8). The Y142X mutation produces no detectable protein because it occurs early in the transcript and is predicted to initiate nonsense-mediated mRNA decay. Mutations in the catalytic domain (L253F) interfere with the autocatalytic cleavage of the protein. In cells expressing the PCSK9-253F, the amount of mature protein was reduced compared to that in cells expressing PCSK9-WT, suggesting that the mutation inhibits autocatalytic cleavage. The L253F mutation is near the catalytic triad (PCSK9 is a serine protease), therefore it might disrupt the active site (8). Inasmuch as autocatalytic cleavage of PCSK9 is required for export of the protein out of the ER, the L253F mutation delays transport of PCSK9 from the ER to the cell surface. The nonsense mutation (C679X) in PCSK9, which truncates the protein by 14 amino acids, did not interfere with protein processing, but the mature protein accumulates in the cells and none is secreted, suggesting that the protein is cleaved normally but is misfolded and is retained in the ER (8, 9).
The mechanism by which PCSK9 causes the degradation of the LDLR has not been fully elucidated. However, it is clear that the protease activity of PCSK9 is not required for LDLR degradation (10, 11). Li et al. (10) have co-expressed the prodomain and the catalytic domain in trans, and showed that the secreted PCSK9 was catalytically inactive, yet it is functionally equivalent to the wild-type protein in lowering cellular LDL uptake and LDLR levels. Similar studies were also reported by McNutt et al. (11). Furthermore, Zhang et al. (12) has mapped PCSK9 binding to the EGF-A repeat of the LDLR, and showed that such binding decreases the receptor recycling and increases its degradation. They also reported that binding to EGF-A domain was calcium-dependent and increased dramatically with reduction in pH from 7 to 5.2. Recently, Kwon et al. (13) determined the crystal structure of PCSK9 in complex with the LDLR-EGF-AB (EGF-A and EGF-B). The structure shows a well defined EGF-A domain, but the EGF-B domain is disordered and absent from their electron density map. The EGF-A domain binds to the PCSK9 catalytic domain at a site distant from the catalytic site, and makes no contact with either the C-terminal domain or the prodomain (14).
Several strategies have been proposed for targeting PCSK9 (15). Strategy 1: mRNA knockdown approaches include the use of antisense oligonucleotides or RNAi. Antisense oligonucleotides administered to mice reduced PCSK9 expression by >90% and lowered plasma cholesterol levels by 53% (16). A single intravenous injection of an RNAi delivered in lipidoid nanoparticles to cynomologous monkeys reduced plasma PCSK9 levels by 70% and plasma LDL-C levels by 56% (17). Strategy 2: is to prevent binding of PCSK9 to the LDLR on the cell surface with a small molecule, a peptide, or an antibody directed against PCSK9. Adding EGF-A fragments to cultured cells inhibits the ability of exogenously added PCSK9 to mediate LDLR degradation. Strategy 3: is to develop small-molecule inhibitors of the PCSK9 processing. Despite evidence that the catalytic activity of PCSK9 is not required for LDLR degradation (11), an intracellular inhibitor of PCSK9 catalytic activity should be effective, since autocatalytic processing of PCSK9 is required for secretion of the protein from the ER. Following its synthesis, PCSK9 undergoes an autocatalytic cleavage reaction that clips off the prodomain, but the prodomain remains attached to the catalytic domain (18, 19). The autocatalytic processing step is required for the secretion of PCSK9 (20), likely because the prodomain serves as a chaperone and facilitates folding. The continued attachment of the prodomain partially blocks the substrate binding pocket of PCSK9 (18, 19). McNutt et al. (21) demonstrated that antagonism of secreted PCSK9 increases LDLR expression in HepG2 cells. They show that an FH-associated LDLR allele (H306Y) that results in a gain-of-function mutation is due to an increase in the affinity of PCSK9 to the LDLR, which would lead to enhanced LDLR destruction, and decreased plasma LDL-C clearance. Furthermore, they were able to show elegantly that blocking the secreted PCSK9 with LDLR (H306Y) subfragment resulted in an increase in the level of LDLR in cultured HepG2 cells. Therefore, PCSK9 acts as a secreted factor to cause LDLR degradation, and a small molecule inhibitor that interferes with the autocatalytic process should decrease the amount of mature secreted PCSK9. This invention relates to identification of small molecules that down-regulate the function of PCSK9 using Strategy 3.
Recently (22-24), Regeneron/Sanofi and Amgen have reported Phase II proof-of-concept data that validate the blocking of PCSK9 with a monoclonal antibody as a strategy for lowering LDL-C in patients not controlled on standard statin therapy. They reported that a single injection of their drug, called REGN727, slashed LDL levels by more than 60% in clinical trial. Their approach follows Strategy 2 using antibodies instead of small molecules. This Strategy 2 is also being pursued by Merck, Novartis and Pfizer, while Strategy 1 is being pursued by Alnylam, Idera and Santaris (25).